In this months high times, there is a question and answer area with Ed Rosenthal. He mentions a study done in the 70's by a Dr Carlton. He found THC levels at the highest just before dawn. Rosenthal surmises that 24-36 period of darkness would increase the THC levels more than 12 hours. I just try and harvest mine an hour or two before the lights come on. I am already using two rooms.. I cant afford a dark room also.